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Simon Peyton Jones authored
Issue #17056 revealed that we were sometimes building a case expression whose type field (in the Case constructor) was bogus. Consider a phantom type synonym type S a = Int and we want to form the case expression case x of K (a::*) -> (e :: S a) We must not make the type field of the Case constructor be (S a) because 'a' isn't in scope. We must instead expand the synonym. Changes in this patch: * Expand synonyms in the new function CoreUtils.mkSingleAltCase. * Use mkSingleAltCase in MkCore.wrapFloat, which was the proximate source of the bug (when called by exprIsConApp_maybe) * Use mkSingleAltCase elsewhere * Documentation CoreSyn new invariant (6) in Note [Case expression invariants] CoreSyn Note [Why does Case have a 'Type' field?] CoreUtils Note [Care with the type of a case expression] * I improved Core Lint's error reporting, which was pretty confusing in this case, because it didn't mention that the offending type was the return type of a case expression. * A little bit of cosmetic refactoring in CoreUtils
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